Hemostasis is the spontaneous process of arresting bleeding from damaged blood vessels. Upon injury, precapillary vessels contract within seconds, and thrombocytes, or blood platelets, bind to the exposed subendothelial matrix of an injured vessel by a process called platelet adhesion. Platelets also stick to each other in a phenomenon known as platelet aggregation to form stable platelet aggregates that quickly help stop or slow blood outflow from injured vessels.
An intravascular thrombus can result from pathological disturbances of hemostasis, or by the rupture of atherosclerotic plaques. Platelet adhesion and aggregation are critical events in intravascular thrombosis. Activated under conditions of high shear blood flow in diseased vessels or by the release of mediators from other circulating cells and damaged endothelial cells lining the vessel, platelets and other cells accumulate at a site of vessel injury to form a thrombus, and recruit more platelets to the developing thrombus. The thrombus can grow to sufficient size to block off arterial blood vessels. Thrombi can also form in areas of stasis or slow blood flow in veins. Venous thrombi can easily detach portions of themselves, creating emboli that travel through the circulatory system. This process can result in blockade of other vessels, such as pulmonary arteries. Blockages of this sort can result in pathological outcomes such as pulmonary embolism. Thus, arterial thrombi cause serious disease by local blockade, whereas the morbidity and mortality associated with venous thrombi arise primarily after distant blockade, or embolization. Conditions associated with pathological thrombus formation include venous thromboembolism, thrombophlebitis, deep vein thrombosis, arterial embolism, coronary and cerebral arterial thrombosis, unstable angina, myocardial infarction, stroke, transient ischemic attack, cerebral embolism, renal embolism and pulmonary embolism.
A number of converging pathways lead to platelet aggregation. Whatever the initial stimulus, the final common event is crosslinking of platelets by binding of fibrinogen to a membrane binding site, glycoprotein IIb/IIIa (GP IIb/IIIa, also known as integrin αIIbβ3). Antagonists of the GP IIb/IIIa receptor have been shown to produce potent antithrombotic effects (Ali, U.S. Pat. No. 6,037,343; Duggan, et al., U.S. Pat. No. 6,040,317). GP IIb/IIIa antagonists include function-blocking antibodies like Abciximab (ReoPro®), cyclic peptides and peptidomimetic compounds (The EPIC investigators; Califf, R. M., coordinating author, New Engl. J. Med. 330: 956-961 (1994); The IMPACT-II investigators, Lancet 349:1422-1428 (1997); The RESTORE investigators, Circulation 96: 1445-1453 (1997)). The clinical efficacy of some of these newer drugs, such as Abciximab, is impressive, but recent trials have found that these approaches are associated with an increased risk of major bleeding, sometimes necessitating blood transfusion (The EPIC investigators; Califf, R. M., coordinating author, New Engl. J. Med. 330: 956-961 (1994)). Also, administration of this class of antiplatelet agent appears to be limited to intravenous methods.
Thrombin can produce platelet aggregation independently of other pathways but substantial quantities of thrombin are unlikely to be present without prior activation of platelets by other mechanisms. Thrombin inhibitors, such as hirudin, are highly effective antithrombotic agents. However, functioning as both antiplatelet and anti-coagulant agents, thrombin inhibitors again may produce excessive bleeding (The TIMI 9a Investigators, Circulation, 90: 1624-1630 (1994); The GUSTO IIa Investigators, Circulation, 90: 1631-1637 (1994); Neuhaus, et al., Circulation, 90: 1638-1642 (1994)).
Various antiplatelet agents have been studied as inhibitors of thrombus formation. Some agents such as aspirin and dipyridamole have come into use as prophylactic antithrombotic agents, and others have been the subjects of clinical investigations. To date, therapeutic agents such as the disintegrins, and the thienopyridines ticlopidine (TICLID®) and clopidogrel (PLAVIX®) have been shown to have utility as platelet aggregation inhibitors, although they can produce a substantial number of side effects and have limited effectiveness in some patients. (Hass, et al., N. Engl. J. Med., 321: 501-507 (1989); Weber, et al., Am. J. Cardiol. 66: 1461-1468 (1990); Lekstrom and Bell, Medicine 70: 161-177 (1991)). In particular, the use of the thienopyridines in antiplatelet therapies has been shown to increase the incidence of potentially life threatening thrombotic thrombocytopenic purpura (Bennett, et al., N. Engl. J. Med, 342: 1771-1777 (2000)). Aspirin, which has a beneficial effect on the inhibition of platelet aggregation (Antiplatelet Trialists' Collaboration, Br. Med. J. 308: 81-106 (1994); Antiplatelet Trialists' Collaboration, Br. Med. J. 308: 159-168 (1994)), acts by inhibiting the synthesis of prostaglandins. Its well-documented, high incidence of gastric side effects, however, limits its usefulness in many patients. In addition, aspirin resistance has been observed in some individuals (McKee, et al., Thromb. Haemost. 88: 711-715 (2002)).
Many studies have demonstrated that adenosine 5′-diphosphate (ADP) plays a key role in the initiation and progression of arterial thrombus formation (Bemat, et al., Thromb. Haemostas. 70: 812-826 (1993)); Maffrand, et al., Thromb. Haemostas. 59: 225-230 (1988); Herbert, et al., Arteriosc. Thromb. 13: 1171-1179 (1993)). ADP induces inhibition of adenylyl cyclase and modulation of intracellular signaling pathways such as activation of phosphoinositide-3 kinase (PI3K), influx and mobilization of intracellular Ca+2, secretion, shape change, and platelet aggregation (Dangelmaier, et al. Thromb Haemost. 85: 341-348 (2001)). ADP-induced platelet aggregation is triggered by its binding to specific receptors expressed in the plasma membrane of the platelet. There are at least three different P2 receptors expressed in human platelets: P2X1, P2Y1, and P2Y12. The P2X1 receptor is a ligand-gated cation channel that is activated by ATP, resulting in a transient influx of extracellular calcium. This receptor has been implicated in the regulation of platelet shape change, and recent evidence suggests its participation in thrombus formation in small arteries under high shear forces. (Jagroop, et al., Platelets 14:15-20 (2003); Hechler, et al., J. Exp. Med. 198: 661-667 (2003)). The P2Y1 receptor is a G protein-coupled receptor that is activated by ADP, and is responsible for calcium mobilization from intracellular stores, platelet shape change and initiation of aggregation. The P2Y12 receptor, also referred to as the P2Yac and P2T receptor, is a G protein-coupled receptor that is activated by ADP and is responsible for inhibition of adenylyl cyclase and activation of PI3K. Activation of P2Y12 is required for platelet secretion and stabilization of platelet aggregates (Gachet, Thromb. Haemost. 86: 222-232 (2001); Andre, et al., J. Clin. Invest., 112: 398-406 (2003)).
ADP-induced platelet aggregation requires the simultaneous activation of both P2Y1 and P2Y12 receptors, and therefore, aggregation can be inhibited by blockade of either receptor. Several authors have demonstrated that ADP-induced aggregation is inhibited in a concentration-dependent manner by analogues of adenosine triphosphate (ATP). ATP, itself, is a weak and nonselective, but competitive, P2Y1 and P2Y12 receptor antagonist. Ingall, et al. (J. Med. Chem. 42: 213-220 (1999)) have reported that modification of the polyphosphate side chain of ATP along with substitution of the adenine moiety at the C2-position, resulted in compounds that inhibited the P2T receptor (or P2Y12 receptor). Zamecnik (U.S. Pat. No. 5,049,550) has disclosed a method for inhibiting platelet aggregation by administration of a diadenosine tetraphosphate-like compound, App(CH2)ppA. Kim and Zamecnik (U.S. Pat. No. 5,681,823) have disclosed P1, P4-(dithio)-P2, P3-(monochloromethylene)-5′,5′″-diadenosine-P1, P4-tetraphosphate as an antithrombotic agent.
Nucleotide P2Y12 antagonists have been developed, however, there is still a need for compounds that have improved oral bioavailability and blood stability.
Thienopyridines, ticlopidine and clopidogrel react covalently with the P2Y12 receptor and produce irreversible platelet inhibition in vivo (Quinn and Fitzgerald, Circulation 100: 1667-1672 (1999); Geiger, et al., Arterioscler. Thromb. Vasc. Biol. 19: 2007-2011 (1999); Savi, et al., Thromb Haemost. 84: 891-896 (2000)). Patients treated with thienopyridines usually require 2-3 days of therapy to observe significant inhibition of platelet aggregation, however, and maximal inhibition usually is observed between 4 to 7 days after initiation of treatment. Also, the platelet inhibitory effect of thienopyridines persists up to 7-10 days after the therapy is discontinued, and both ticlopidine and clopidogrel produce a significant prolongation of the bleeding time (from 1.5 to 2-fold over control). Because of the prolonged effect of thienopyridines, these drugs need to be discontinued for 7 to 10 days prior to elective surgery, leaving the patient unprotected from a possible thrombotic event during that period. Recently, the association of thienopyridine treatment with events of thrombotic thrombocytopenic purpura has been reported (Bennett, et al., N. Engl. J. Med. 342: 1773-1777 (2000); Bennett, et al., Ann. Intern. Med. 128: 541-544 (1998)).
Derivatives of 5,7-disubstituted-1,2,3-triazolol[4,5-d]pyrimidin-3-yl-cyclopentanes and -tetrahydrofurans have been disclosed as antagonists of the P2T-(or P2Y12) receptor on platelets (Cox, et al., U.S. Pat. No. 5,747,496, and related patents; Bonnert, et al., U.S. Pat. No. 6,297,232; WO 98/28300; Brown, et al., WO 99/41254; WO 99/05144; Hardern, et al. WO 99/05142; WO 01/36438; and Guile, et al. WO 99/05143) for use in the treatment of platelet aggregation disorders.
Guile, et al. (WO 00/04021) disclose the use of triazolo[4,5-d]pyrimidine compounds in therapy. Brown, et al. (U.S. Pat. No. 6,369,064) disclose the use of triazolo(4,5-d)pyrimidine compounds in the treatment of myocardial infarction and unstable angina. Dixon, et al. (WO 02/096428) disclose the use of 8-azapurine derivatives in combination with other antithrombotic agents for antithrombotic therapy. Springthorpe discloses AZD6140 as a potent, selective, orally active P2Y12 receptor antagonist which is now in Phase I clinical trials (Abstracts of Papers, 225th ACS National Meeting, New Orleans, La.; March, 2003; MEDI-016). WO 02/016381 discloses a method of preventing or treating diseases or conditions associated with platelet aggregation using mononucleoside polyphosphates and dinucleoside polyphosphates.
There is still a need in the areas of cardiovascular and cerebrovascular therapeutics, and in blood product preparation, purification, and storage, for selective, reversible inhibitors of platelet activation, which can be used in the prevention and treatment of thrombi or other aggregation-related problems.